Honestly, I feel like they’re both different enough characters that you could do an RO line for both of them.
Daisetsu is such a cinnamon roll, I just wanna wrap him in a carpet and protect him rather than let him have it the other way around. He’s so sweet. He reminds me of a baby chick just shedding its downy feathers; so ready for the world to break him.
Isamu’s different. He’s clearly been through some stuff since the night he first gave us some konpeito. He’s older too, so I think he’d have more of a story to tell than Daisetsu.
But, considering how many RO symbols I’ve already seen for this story, I can understand wanting to have one less. But, in case you change your mind, I trust your abilities as an author to be able to make these two potentially similar characters very different yet equally interesting.
Should Daisetsu be chosen, I’d love to see the kind of dynamic Isamu can have with the MC. As you’ve said, he has a bit of an older brother thing going on. So, I can kind of picture him having a soft spot for the MC and it’d be fun to see how he interacts with them, only to see him act differently around others. I’m sure he’d be a fun friend to see, even if only occasionally.
Should Isamu be chose, I can definitely see the MC and Daisetsu still being close. Should they be so inclined, the MC could try and get Daisetsu to loosen up a little. Maybe be the Red Oni to his Blue Oni? …y’know, since he’s so sweet and pure, I can actually imagine a female MC teasing him in a somewhat sexual manner (e.g. Starting to strip in front of him) whether it be as an RO or just as a friend because his reaction would be AWESOME.
But… if I had to choose…? I guess I’d pick Isamu. He’s a little more jaded, so he’d certainly be an interesting route. But Daisetsu’s so precious…! Honestly, I don’t know why I’m getting so antsy about this. I’m gonna romance the Tanuki or Kitsune. xD